Improved sensors are needed that can operate in harsh environments for the next generation of technologies for higher efficiency, lower emission fossil-fueled power plants including oxy-fuel combustion processes for carbon capture and sequestration and coal gasification to produce syngas which can be converted to electrical power using solid-oxide fuel cells or gas turbines. Improved harsh environment sensors and controls would also enable significant gains in energy efficiency for the existing fleet of coal-fired power plants and a number of major domestic manufacturing industries. In particular, chemical sensors capable of operating at elevated temperatures in highly reducing, oxidizing, and/or corrosive environments can be leveraged across a broad range of applications including coal gasification, combustion turbines, solid oxide fuel cells, and advanced boiler systems.
Optical sensors are of increasing interest for a wide range of embedded sensing applications due to a number of inherent advantages as compared to other sensor technologies including the ability to monitor several different optical properties of a selected sensing material (transmission, reflection, luminescence). While there is a large body of existing work on electrical responses of semiconducting materials for applications in chemi-resistive based gas sensing, corresponding optical responses are not as well understood thereby providing very limited guidance for their applications in optical-based gas sensing. Material systems with useful optical responses specifically tailored for the application of interest are therefore required.
Metal oxides such as WO3 have been utilized as optical sensors for H2 while other metal oxides such as NiO and Co3O4 have been explored for optical sensing of reducing gases such as CO. However, these materials suffer from limited temperature stability in highly reducing conditions and typical dynamic ranges of measured output signals based on absorbance or reflectance have limited their practical use in a gas sensing instrument. See e.g. Ando, “Recent advances in optochemical sensors for the detection of H2, O2, O3, CO, CO2 and H2O in air,” Trends in Analytical Chemistry 25(10) (2006); see also Korotcenkov, “Metal oxides for solid-state gas sensors: What determines our choice?” Materials Science and Engineering B 139 (2007). Incorporation of noble metals such as gold nanoparticles into these metal oxides has generally been employed to enable responses that are suitable for practical gas sensing. See e.g., Schleunitz et al., “Optical gas sensitivity of a metal oxide multilayer system with gold-nano-clusters,” Sensors and Actuators B 127 (2007); see also Gaspera et al., “CO optical sensing properties of nanocrystalline ZnO—Au films: Effect of doping with transition metal ions,” Sensors and Actuators B 161 (2012); see also Gaspera et al., “Enhanced optical and electrical gas sensing response of sol-gel based NiO—Au and ZnO—Au nanostructured thin films,” Sensors and Actuators B 164 (2012); and see Ando et al., “Combined effects of small gold particles on the optical gas sensing by transition metal oxide films,” Catalysis Today 36 (1997). In other cases, metal oxides such as ZnO with various dopants have been utilized and absorbance changes have been noted for gases such as ammonia, methanol, and ethanol, however the mechanism has generally been attributed to the adsorption of oxygen molecules at the metal oxide surface and the dopant was utilized to enhance catalytic activity, and correspondingly measurement temperatures have been limited to below about 100° C. The time constants for the measured responses also tend to be prohibitively long such that they are not practical for a gas sensing device. See e.g., Renganathan et al., “Gas sensing properties of a clad modified fiber optic sensor with Ce, Li and Al doped nanocrystalline zinc oxides,” Sensors and Actuators B 156 (2011). Dopants such as CuO have also been employed with metal oxides such as ZrO2 in order to provide sensing through reversible red-ox reactions, however such approaches can suffer from instability under high temperature and/or high reducing agent concentrations. See e.g., Remmel et al., “Investigation on nanocrystalline copper-doped zirconia thin films for optical sensing of carbon monoxide at high temperature,” Sensors and Actuators B 160 (2011).
Weak dynamic range of optical responses of high temperature stable metal oxides to changing gas atmospheres has generally required investigators to amplify the response by applying them to optical fibers with fiber bragg gratings. For example, low electronic conductivity perovskite based oxides such as terbium doped strontium cerate have been integrated with long period fiber gratings and have demonstrated useful and selective responses to H2 at elevated temperatures. By periodically modifying the refractive index of the core of the optical fiber, the interaction with a sensing layer can be enhanced by orders of magnitude. See e.g. Tang et al., “Acidic ZSM-5 zeolite-coated long period fiber grating for optical sensing of ammonia,” J. Mater. Chem. 21 (2011); see also Jiang et al., “Multilayer fiber optic sensors for in situ gas monitoring in harsh environments,” Sensors and Actuators B 177 (2013); see also Wei et al, “Terbium doped strontium cerate enabled long period fiber gratings for high temperature sensing of hydrogen,” Sensors and Actuators B 152 (2011); see also Remmel et al., “Investigation on nanocrystalline copper-doped zirconia thin films for optical sensing of carbon monoxide at high temperature,” Sensors and Actuators B 160 (2011); see also Tang et al., “Proton-Conducting Nanocrystalline Ceramics for High-Temperature Hydrogen Sensing”, Metallurgical and Materials Transactions E 48 (2014). However, fiber bragg gratings typically exhibit an inherent temperature instability above 500° C. regardless of the sensing layer employed and dramatically increase device cost and complexity.
It would be advantageous to utilize a method that employs a class of metal oxides with relatively large and gas-sensitive optical absorption across a broad wavelength spectrum to maximize compatibility with the broadest possible range of optical sensor devices. Two primary wavelength ranges of interest for designing optical sensor devices include the visible range (˜400-700 nm) and the near-infrared telecommunications wavelength range (˜1500-1600 nm) for which a broad array of optical components, sources, and devices are commercially available and relatively inexpensive. It would be further advantageous if the class of materials provided adequate optical signal response to changes in chemical compositions to mitigate the need for utilization of advanced sensor designs such as fiber bragg gratings or for incorporation of noble metals, such as gold, platinum, and silver. It would be particularly advantageous if the method of improvement remained effective or even further improved at higher temperatures, in order to avoid the low temperature limitations associated with alternate methodologies. It would be further advantageous if the increased response of the metal oxide material could be brought about by relatively well understood processes, such as optimizing material chemistry, doping, optimization of deposition techniques and conditions, and carefully selected elevated temperature pretreatments prior to deployment for chemical sensing applications. It would be further advantageous if the material response demonstrated reversibility under high temperature conditions of interest.
Presented here is a method of detecting changes in the chemical composition of a gaseous stream by utilizing the optical response of an electronically conducting perovskite-based oxide material. The unique optical properties of the perovskite-based oxides are well known to derive from their electronic band structure which is intimately linked to the underlying crystal structure. Perovskite-based oxide materials with relatively high electronic conductivity such as La1-xSr, CoO3, La1-xSrxMnO3, LaCrO3, LaNiO3, La1-xSrxMn1-yCryO3, SrFeO3, SrVO3, La-doped SrTiO3, Nb-doped SrTiO3, and SrTiO3-δ have been observed to display a relatively large and broad-band optical absorption across the entire wavelength range from the ultraviolet to the near-infrared. Suitable optimization of high temperature stable electronically conducting perovskite-based oxides for elevated temperature gas sensing applications can be achieved through (1) composition modification, (2) doping, (3) synthesis technique and details, and (4) post-synthesis pretreatments at elevated temperatures among others. The surprisingly effective method utilized within this disclosure provides a means whereby electronically conducting perovskite-based metal oxides are employed to generate improved signals under gaseous atmospheres which experience varying concentrations of reducing and oxidizing agents. In contrast with electronically conductive perovskite-based oxides, the optical absorption of common semiconductor metal oxides employed in chemi-resistive sensing applications such as ZnO, TiO2, and SnO2 are typically limited to wavelengths below the so-called band-edge and are associated with interband electronic transitions that are not strongly sensitive to changes in ambient gas atmospheres thereby making them less advantageous for high temperature optical gas sensing applications despite a well-known and well-characterized response of the electrical resistivity of such conventional metal oxide systems.
These and other objects, aspects, and advantages of the present disclosure will become better understood with reference to the accompanying description and claims.